Light control material is a material having a light transmittance variable in accordance with the magnitude of an applied electric field, so as to be adjustable in the quantity of incident light. A light control material is obtained, for example, by dispersing, into a resin matrix, a light control suspension wherein light control particles responsible to an electric field are dispersed. Known is a light control film wherein a light control layer obtained by making the light control material into a film form is sandwiched between transparent electroconductive substrates.
This light control film is a film using a light control layer in which fine droplets of a light control suspension wherein light control particles are dispersed are dispersed in a resin matrix cured by irradiation with ultraviolet rays. The light control particles in the light control film undergo Brownian movement in the state that no electric field is applied thereto, thereby adsorbing, scattering or reflecting light; thus, the film cannot transmit incident light onto the film. When an electric field is applied thereto, the light control particles are polarized, thereby being arranged in directions parallel to each other in accordance with the electric field. As a result, the film starts transmitting incident light onto the film. In such a way, the film adjusts the transmission quantity of light by the response of its light control particles to an electric field (see, for example, Patent Document 1).
As the light control particles, particles described in Patent Document 2 are known. In recent years, however, it has been considered that light control particles disclosed in Patent Document 3 are the closest to light control particles to be commercialized since the particles are excellent in heat resistance and weather resistance.
Out of methods for manufacturing the light control particles, for example, a method disclosed in Patent Document 4 is a preferred manufacturing method since the method is the best in productivity. However, water significantly affects the particle size and the shape of the light control particles; thus, it is indispensable to analyze strictly the total amount of water present in the reaction system when the particles are produced. About, for example, cellulose nitrate, which is added in order to prevent the light control particles from aggregating and other purpose in the manufacturing method, is a commercially available product and generally contains water in an amount up to about 5% by mass. The water amount therein needs to be precisely measured. About calcium iodide, as which a dehydration product of a cheap hydrate thereof is frequently used, it is necessary to measure the amount of water remaining therein precisely.
In order to prepare the light control particles to have a particle size and a shape optimal for light control material, it is necessary to adjust strictly the amount of water present in the reaction system when the particles are manufactured.
As described above, in the method for manufacturing light control particles disclosed in Patent Document 4, it is essential for preparing light control particles having a particle size and a shape optimal for light control material to adjust strictly the amount of water present in the system when particles are manufactured. Furthermore, narrow is a permissible range of the water amount required for obtaining light control particles having a desired particle size and shape. It is therefore very difficult to adjust the water amount when light control particles are mass-produced. Thus, this method has a problem in industrial applicability, so as not to be easily industrialized.